The present invention relates to a color cathode ray tube.
The resolution of a color cathode ray tube largely depends on the dimension and shape of spots (beam spots) of electron beams formed on a phosphor surface. To obtain the high resolution, electrodes of an electron gun must be constituted such that beam spots possibly having the smallest diameter and a circular shape are formed.
On the other hand, the diameter of electron beams which pass through a main lens of the electron gun becomes larger corresponding to the increase of the beam current and the diameter of the beam spots also becomes larger depending on the spherical aberration of the main lens. When the diameter of the main lens is increased by enlarging the diameter of a neck portion which accommodates the electron gun (the neck diameter), the diameter of the beam spot can be made small. In this case, however, the deflection electric power is increased. Japanese Laid-Open Patent Publication 103752/1983 discloses a technique which can minimize the spherical aberration by setting the diameter of the main lens as large as possible without enlarging the neck diameter.
FIG. 10 is a cross-sectional schematic view of an in-line-type electron gun of the prior art taken along a tube axis thereof. This electron gun includes an electron beam generating part which is comprised of a cathode 1 (a center cathode) encasing a heater 1xe2x80x2, a control electrode 2 and an accelerating electrode 3, a focusing electrode 4 provided with a plate electrode 5 therein, and an anode 6 provided with a plate electrode 7 therein. Here, the above-mentioned respective electrodes and anode are formed of tubular electrodes having an elliptical or oblong cross section.
FIG. 11A is a plan view of the plate electrode 5 which is provided to the inside of the focusing electrode 4 and FIG. 11B is a plan view of the plate electrode 7 provided to the inside of the anode 6. The focusing electrode 4 has a single opening 4xe2x80x2 through which three electron beams are made to pass. The plate electrode 5 provided to the inside of the focusing electrode 4 includes three electron beam apertures consisting of a center beam aperture 5c and two side beam apertures 5s. The anode 6 also has a single opening 6xe2x80x2 through which three electron beams are made to pass through. The plate electrode 7 provided to the inside of the anode 6 includes one center beam aperture 7c and two semi-elliptical side beam portions 7s. 
In the electron gun having the above-mentioned constitution, thermal electrons emitted from three cathodes 1 heated by the heater 1xe2x80x2 (here, only the heater 1xe2x80x2 and the cathode 1 for the center beam shown ) are sucked to the control electrode 2 side due to a positive voltage Vg2 of 400-1000 V applied to the accelerating electrode 3 to form three electron beams. The three electron beams pass through an opening portion of the control electrode 2, subsequently pass through the opening portion of the accelerating electrode 3, and thereafter, pass through a main lens which is formed in an opposing gap defined between the focusing electrode 4 and the anode 6 while being accelerated by a positive voltage applied to the focusing electrode 4 and the anode 6.
Here, the electron beams receive a slight focusing action before entering the main lens due to a prefocusing lens formed between the accelerating electrode 3 and the focusing electrode 4 to which a focusing voltage Vf of approximately 5-10 kV is applied. An anode voltage Eb of approximately 20-35 kV is applied to the anode 6 through a shield cup 8. Due to the main lens formed by a potential difference between the focusing electrode 4 and the anode 6, the electron beams focus on a phosphor screen and form beam spots on the phosphor screen.
As described above, since the single electron beam openings 4xe2x80x2, 6xe2x80x2 of the main lens electrode, that is, of the focusing electrode 4 and the anode 6 are large, an electric field of a main lens electrode opposing portion deeply penetrates into the inside of the main lens electrode and hence, it becomes possible to obtain an advantageous effect that the opening portion is substantially enlarged, that is, the diameter of the main lens is enlarged compared to a normal cylindrical lens. The enlargement of the diameter of the main lens can reduce the spherical aberration of the main lens and can minimize the enlargement of beam spots caused by the spherical aberration so that excellent focusing characteristics can be obtained.
However, even with respect to the electron gun having such a constitution, the substantial enlargement of the diameter of the main lens is restricted by the dimension between points on the loci of three electron beams and a point on the above-mentioned single electron beam aperture of the main lens electrode which come closest to each other.
That is, among the distances from the end of the single beam aperture in the in-line direction (in the horizontal direction) or from the end of the single electron beam aperture in the direction perpendicular to the in-line direction to the loci of respective electron beams, or the distance from the end of the single electron beam aperture in the in-line direction to the loci of both side beams among electron beams, the shorter one corresponds to the radius of the main lens. Accordingly, there has been a problem with respect to the conventional electron gun that the effective diameter of the main lens is restricted.
In a color cathode ray tube of the present invention, an intermediate electrode to which a voltage set to a middle value between a focusing voltage and an anode voltage is applied is arranged between a focusing electrode and an anode of an in-line type electron gun, the intermediate electrode has a single opening having the diameters in the horizontal direction (in-line direction) greater than the diameter in the direction perpendicular to the horizontal direction and allowing three electron beams to pass therethrough and is provided with a plate electrode which has three electron beam apertures for allowing three electron beams to pass therethrough therein. In the inside of the focusing electrode of the electron gun of the color cathode ray tube according to the present invention, a plate electrode having three electron beam apertures is arranged. The relationship between the length Lc which is a value obtained by adding the diameters in the horizontal direction of the three electron beam apertures and the lengths in the horizontal direction of bridges disposed between neighboring electron beam apertures in the inside of the focusing electrode and the horizontal length Lm which is a value obtained by adding the diameters in the horizontal direction of the three electron beam apertures and the lengths in the horizontal direction of bridges disposed between neighboring electron beam apertures of the plate electrode in the inside of the intermediate electrode is set to Lc greater than Lm.
Due to such a constitution, the diameter of the main lens can be enlarged and the aberration can be reduced and hence, the color images having a high definition can be displayed.
Further, according to the color cathode ray tube of the present invention, the relationship between the length La which is a value obtained by adding the diameters in the horizontal direction of the three electron beam apertures and the lengths in the horizontal direction of bridges disposed between neighboring electron beam apertures of the plate electrode in the inside of the anode and the horizontal length Lm which is a value obtained by adding the diameters in the horizontal direction of the three electron beam apertures and the lengths in the horizontal direction of bridges disposed between neighboring electron beam apertures of the plate electrode in the inside of the intermediate electrode is set to La greater than Lm.
Further, according to the color cathode ray tube of the present invention, by setting the relationships between the above-mentioned La, Lm and Lc to Lc greater than Lm and La greater than Lm, the diameter of the main lens can be enlarged and the aberration can be reduced so that color images of high definition can be displayed.